Orangutans modify facial displays depending on recipient attention.

Primate facial expressions are widely accepted as underpinned by reflexive emotional processes and not under voluntary control. In contrast, other modes of primate communication, especially gestures, are widely accepted as underpinned by intentional, goal-driven cognitive processes. One reason for this distinction is that production of primate gestures is often sensitive to the attentional state of the recipient, a phenomenon used as one of the key behavioural criteria for identifying intentionality in signal production. The reasoning is that modifying/producing a signal when a potential recipient is looking could demonstrate that the sender intends to communicate with them. Here, we show that the production of a primate facial expression can also be sensitive to the attention of the play partner. Using the orangutan (Pongo pygmaeus) Facial Action Coding System (OrangFACS), we demonstrate that facial movements are more intense and more complex when recipient attention is directed towards the sender. Therefore, production of the playface is not an automated response to play (or simply a play behaviour itself) and is instead produced flexibly depending on the context. If sensitivity to attentional stance is a good indicator of intentionality, we must also conclude that the orangutan playface is intentionally produced. However, a number of alternative, lower level interpretations for flexible production of signals in response to the attention of another are discussed. As intentionality is a key feature of human language, claims of intentional communication in related primate species are powerful drivers in language evolution debates, and thus caution in identifying intentionality is important.

Paedomorphic facial expressions give dogs a selective advantage.

How wolves were first domesticated is unknown. One hypothesis suggests that wolves underwent a process of self-domestication by tolerating human presence and taking advantage of scavenging possibilities. The puppy-like physical and behavioural traits seen in dogs are thought to have evolved later, as a byproduct of selection against aggression. Using speed of selection from rehoming shelters as a proxy for artificial selection, we tested whether paedomorphic features give dogs a selective advantage in their current environment. Dogs who exhibited facial expressions that enhance their neonatal appearance were preferentially selected by humans. Thus, early domestication of wolves may have occurred not only as wolf populations became tamer, but also as they exploited human preferences for paedomorphic characteristics. These findings, therefore, add to our understanding of early dog domestication as a complex co-evolutionary process.